Such seals can also include a secondary anti-dust lip, of the same material as the body and projecting therefrom, as well as a coating or covering of the same material on a bearing surface of the frame which presses on the shaft on which the ring is positioned.
Examples of manufacture of such rings are described in German patent application No. 33 09 538.
Such seal rings have certain disadvantages.
First it should be noted that the shearing stress which occurs at the interface between the elastomer body and the seal lip of fluorine containing resin is significant and causes the resin lip to unadhere and separate from the peripheral edge of the body, leading to rapid deterioration of the seal.
In addition, the region of deflection of the fluorine-containing resin lip is relatively indeterminable, so that it is not possible to precisely control the pressure of the lip on the facing seal bearing surface, and it is not possible to control precisely the radial pressure of the ring.